1. Field of the Invention
This invention concerns a dielectric-barrier discharge lamp device.
2. Description of Related Art
In recent years, technology has developed for treating metals,glass and other materials by illuminating the item to be treated with vacuum ultraviolet radiation at wavelengths up to 200 nm, thus allowing the vacuum ultraviolet radiation and the ozone produced thereby to operate on the item to be treated. Examples are cleaning treatment technology that removes organic pollutants adhered to the surface of the item to be treated, and oxide film formation technology that forms an oxide film on the surface of the item to be treated.
The lamps conventionally used to provide such treatment have been low-pressure mercury lamps that emit vacuum ultraviolet radiation at a wavelength of 185 nm, which is the resonant line of mercury. In recent times, dielectric-barrier discharge lamps have come to be used. These are lamps that produce excimer emissions by containing a gas for excimer emissions in a discharge vessel made up of a dielectric and bringing about a dielectric-barrier discharge (also called xe2x80x9cozonizer dischargexe2x80x9d or xe2x80x9csilent discharge.xe2x80x9d See Discharge Handbook, Association of Electrical Studies, rev. ed. June 1989, p. 263).
Such dielectric-barrier discharge lamps are described in, for example, U.S. Pat. No. 4,945,290 (Japanese Kokai Patent H1-144560). That patent document describes a dielectric-barrier discharge lamp in which a hollow-cylinder-shaped discharge space, made up of quartz glass of which at least a part is dielectric, is filled with a gas for excimer emissions.
A problem of dielectric-barrier discharge lamps of that type is that the lighting efficiency of the lamp (the ratio of area lighted to input power) decreases as the power input to the lamp is increased. The cause is thought to be that the temperature of the gas in the lamp increases with the input power, and the lighting efficiency decreases as a result.
There is an additional problem in that the increase of gas temperature decreases the transmissivity of the quartz glass. For example, the transmissivity at a wavelength of 172 nm is about 85% at 25xc2x0 C., but it falls to about 83% at 100xc2x0 C. and about 73% at 300xc2x0 C.
There is a further problem in that the increased temperature of the lamp lowers the insulator fracture voltage of the quartz glass, and so it is possible for the lamp to fracture and leak. Depending on the application, it is often necessary to increase the input power in order to raise the light output. For that reason, it becomes necessary to reduce the gas temperature by cooling the lamp itself.
FIG. 3 is an explanatory drawing of a conventional dielectric-barrier discharge lamp device fitted with a cooling mechanism. In the drawing, the discharge lamp 1 has two co-axial tubes, an inner tube 2 and an outer tube 3, forming a hollow-cylinder-shaped discharge space 4 between the inner tube 2 and the outer tube 3. The inner tube 2 and the outer tube 3 are made up of a dielectric, at least in part. For example, the inner tube 2 and the outer tube 3 are made up of quartz glass that allows light at a wavelength of 172 nm to pass.
A roughly cylindrical electrode 5 is placed in close contact with the inner surface of inner tube 2. This internal electrode 5 is made up by joining two half cylinders formed by bending aluminum sheets. Around the outer surface of the outer tube 3 is placed an external electrode 6 that allows the light to pass through it. The external electrode 6 comprises a mesh electrode that allows the passage of ultraviolet light. The internal electrode 5 and external electrode 6 are connected to an alternating current power supply that is not illustrated. An inert gas or a mixture of an inert gas and a halogen is placed in the discharge space 4 as a discharge gas.
At each of the ends 1A and 1B of the dielectric-barrier discharge lamp 1, a ring-shaped gasket 7 is located that has a through-hole 7A and that is aligned with the end 1A, 1B of lamp 1. The diameter of the through-holes 7A is the same as the diameter of the inner space P that is formed by the inner tube 2.
A coupler fitting 8 has the gasket 7 on its inner face; by rotating this coupler fitting 8, the gaskets 7 are pressed against the ends 1A, 1B of the dielectric-barrier discharge lamp 1, creating a tight seal between the gaskets 7 and the ends 1A, 1B. Through-holes 8A are formed in the coupler fittings 8 to align with the through-holes 7A in the gaskets 7.
The coupler fittings 8 are held in casings 9 by O-rings 10. This casing 9 is formed with through-holes 9A aligned to allow the passage of a coolant fluid through through-holes 8A.
In other words, the inner space P formed by the inner tube 2 forms a passage along with the through-holes 8A of the coupler fittings 8 and the through-holes 9A of the casing 9. As indicated by the arrows in FIG. 3, the coolant fluid leaving one side of the casing 9 through through-hole 9A then passes through the through-holes 8A and 7A into the inner space P formed by the inner tube 2, to cool the dielectric-barrier discharge lamp 1 from the inner tube 2.
However, the dielectric-barrier discharge lamp 1 is made by welding together the inner tube 2 and the outer tube 3 in order to form the discharge space 4. For that reason, there will be irregularities where the ends 1A, 1B face gaskets 7. That is, when the gaskets 7 are pushed tightly against the ends 1A, 1B, gaps may be left between the gaskets 7 and the ends 1A, 1B if they are not pushed hard enough, and the coolant fluid is liable to leak from those gaps. Also, there is the problem that, if the coolant fluid leaks, it will not be possible to cool the dielectric-barrier discharge lamp 1.
Moreover, vacuum ultraviolet radiation is emitted by the dielectric-barrier discharge lamp 1, and the gaskets 7 are directly illuminated by that vacuum ultraviolet radiation. As a result, there is also the problem that the gaskets 7 deteriorate because of the vacuum ultraviolet radiation. In addition, gaps occur between the gaskets 7 and the ends 1A, 1B in the course of deterioration of the gaskets 7, causing leakage of the coolant fluid from the gaps. Thus, there is a problem in that it becomes impossible to cool the dielectric-barrier discharge lamp 1 if the coolant fluid leaks.
A primary object of this invention is to provide a dielectric-barrier discharge lamp device in which leakage of the coolant fluid used to cool the dielectric-barrier discharge lamp can be reliably prevented, and in which the dielectric-barrier discharge lamp can be reliably cooled.
To achieve the object stated above, this invention provides a dielectric-barrier discharge lamp device having a dielectric-barrier discharge lamp with a hollow-cylinder-shaped discharge space formed by an outer tube that is roughly cylindrical in external shape and a co-axial inner tube, in which the inner tube has a cylindrical tube extension that extends outward from the discharge space, and in which the outer periphery of the end of the tube extension is held tightly by a coupler fitting connected to a guide tube through which a coolant fluid flows.